Soldering technology is widely used in microelectronics and optoelectronics for making electrical and mechanical connections. Conventional soldering methods typically use a liquid flux to promote wetting of solder on a base metal. Flux residues must then be removed from the soldered assembly. Although the residues are generally not electrically conductive, they are corrosive and can cause long term reliability problems. For semiconductor and electronic packaging, the use of liquid flux is further limited due to concerns about contamination of optically active surfaces by organic residue. To meet such considerations, it is generally desirable to employ fluxless soldering during bonding or bumping, typically by using gaseous reducing reagents.
A forming gas comprising a mixture of nitrogen and hydrogen is one type of gaseous reducing reagent utilized to create a reductive atmosphere for soldering die bonds as well as Au/Sn eutectic die bonds. The forming gas mixture can be made of various compositions, but usually, it has a high nitrogen content mixture with the balance being hydrogen gas. Previously, forming gas was applied during brazing at between 650-850° C. to remove oxides and prevent these oxides from forming since the hydrogen would react with any compound capable of creating an undesirable oxide layer. Presently, forming gas (eg. N2&5-15% H2) is widely used in the semiconductor/electronic packaging industry for soft solder die bonding and Au/Sn (Si) eutectic die bonding and other packaging processes because it is convenient and clean.
However, forming gas generally becomes an active gas above a temperature of 350° C. It can be therefore only used as protective gas at temperatures below the active temperature. High H2/H2O pressure ratios or long reaction times are needed for deoxidization with forming gas. Whilst it is convenient and clean, it is insufficiently effective to remove oxide and prevent solder from re-oxidizing. Moreover, there is an initiation temperature below which the reduction is insignificant and above which the reduction process is accelerated. For instance, for a flip chip bonded with 40 μm solder bumps (90Pb/10Sn), the initiation temperature is about 370° C. It is thus necessary to heat the substrate independent of the solder's melting point to temperatures above 350-370° C.
Other solutions focus on the use of various chemical vapors to create reductive atmospheres. In one such solution, formic acid vapor is used as a reaction gas for fluxless soldering. For example, in U.S. Pat. No. 6,344,407 entitled “Method of Manufacturing Solder Bumps and Solder Joints using Formic Acid”, formic acid is used in a reduced pressure atmosphere to form solder bumps on an underlying metal flim of a semiconductor device.
Also, in U.S. Pat. No. 6,207,551 entitled “Method and Apparatus using Formic Acid Vapor as Reducing Agent for Copper Wirebonding”, formic acid vapor is introduced to a bonding zone to remove copper oxide during wirebonding operations to copper metal pads to permit good wirebonds to be achieved. In one aspect of the invention, the concentration of formic acid vapor is controlled by mixing the formic acid vapor with a gas such as nitrogen that does not participate in the reduction chemical reactions.
Formic acid is an effective reactive gas for removing surface oxide on solders, and is active at typical soldering temperatures (about 220° C.) due to its low decomposing temperature (about 150° C.). Also, implementation of processes utilizing formic acid is relatively simple. However, formic acid has certain disadvantages. One disadvantage is that it must be used in a closed environment during fluxless soldering due to its strong corrosiveness, pungent odor and toxicity to humans. Any formic acid residue remaining after the process must also be controlled and safely exhausted, which increases handling complexity.